The present invention is directed to a programmable electronic speed control for controlling the speed of a motor responsive to an ac signal source without altering the amplitude of the ac signal received by the motor.
In particular, the invention is directed to a programmable electronic speed control wherein the speed of the motor is controlled by alternately passing and blocking selectable numbers of cycles of the ac signal to the motor, repetitively, without altering the amplitude of the passed ac signal.
The invention is intended for use in controlling the operation of ac motors of various types at reduced speeds. Such motors would include induction motors of any type, including the shaded pole and permanent split capacitor type, and synchronous (reluctance or hysteresis) motors.
In general, induction motors do not lend themselves to speed control over a wide range of speeds and, where the motor load is other than exponential, at reduced speeds. Synchronous motors have proven impossible to operate at less than synchronous speed.
Presently, low cost induction motor speed controls, such as variable transformers or solid state triac phase controlled switches, achieve motor speed reduction by reducing the actual or apparent motor input voltage. In such speed controls, motor speed tends to vary widely with changing loads unless the motor speed is sensed and a feedback control is utilized to continuously adjust the voltage applied to the motor.
In cases where the motor is operated at a fixed reduced voltage, the motor may fail to start since the starting torque is directly related to the square of the voltage reduction ratio which may be too low. This relationship is exemplified by the following equation: EQU T.sub.R =(V.sub.R /V).sup.2 .times.(T)
where T.sub.R is the starting torque at reduced voltage, (V.sub.R /V) is the voltage reduction ratio, and T is the starting torque at the design voltage V.
In speed control systems employing phase control, reduction of the voltage applied to the motor produces substantial conducted and radiated radio frequency interference, making necessary costly suppression components.
Synchronous motors, in particular, cannot be operated at reduced speeds, less than synchronous speed, according to conventional voltage reduction techniques. The synchronous motor may simply stop, or fail to start, if the voltage applied to the motor is too low for operation at synchronous speed.
The present invention fills a gap in the industry for a low cost speed control which is not subject to the foregoing deficiencies. Speed control is exercised by alternately passing and blocking selectable numbers of cycles of the ac signal applied to the motor. The speed control is programmable and does not require reduction of the amplitude of the ac signal applied to the motor.
Since speed control is achieved by the invention by intermittent application of the full ac signal, torque developed at a controlled speed is comparable to the torque which would be developed by the motor at the same speed but at continuously applied line voltage. Under some transient conditions, the torque developed by the motor at the controlled speed can actually exceed the torque which would be developed by the motor at the same speed at continuously applied line voltage.
The present invention is particularly suited for use with motors which require substantial starting torque and which drive devices that are operated intermittently. The invention, however, can also be employed to control the speed of devices which are operated continuously.